*1.5. Self-Repairing Materials and Coatings*

This group should include AMCs which can provide a spontaneous response to external mechanical, thermal and physical impacts by changing (restoring) the structure and microgeometry. Examples of the implementation of this approach are self-sharpening cutting edges of metalworking tools with blades, for example, equipped with hard laminar or laminated coatings [62]. Nevertheless, the greatest research interest for scientists today is tool materials and coatings that have a shape memory effect (high damping alloys). This effect consists of restoring the original shape of the plastically deformed material, which occurs after its heating to a specific temperature [63,64].

The self-healing phenomenon is associated with martensitic transformations in the crystal lattice of the material, during which an ordered movement of atoms occurs. Martensite in shape memory materials is thermoelastic and consists of crystals in the form of thin

plates that stretch in the outer layers and shrink in the inner layers [65]. The sources of deformation are interphase, twinning, and intercrystalline boundaries. After heating the deformed alloy, internal stresses appear, tending to return the metal to its original shape. The nature of spontaneous recovery depends on the mechanism of the previous exposure and temperature conditions.

Among some known shape memory alloys used in mechanical engineering, an intermetallic compound based on titanium nickelide (Ni-Ti system) has the maximum thermal cyclic strength, but its hardness and heat resistance are insufficient for use as a material for the cutting part of the tool [66]. At the same time, there are examples of using Ni-Ti or analogs as inserts in the construction of cutting tools, particularly for the machining of rocks and building materials. For example, there are known variants of using such materials as compact drives for bringing the worn cutting part of the cutters of drill bits to the working position due to the form restoration of the alloy under a particular thermal effect. There are technical solutions for the manufacture of tool bases from shape memory alloys for fixing diamond elements. In the event of overheating and an unacceptable increase in temperature, the contact area with the workpiece material decreases and the position of the cutting diamond inserts is corrected to minimize their abrasion. Recently, new high-entropy alloys of the Fe-Ni-Co-Al-Nb-Ti type have come to replace traditional alloys of the Ni-Ti system, which have certain advantages and significant prospects for application in tool production. There are examples of assembled tool constructions with CBN inserts including damping elements. This ensures improved vibration resistance and durability of the tool [67].

An analysis of the latest progressive research results shows that the use of highentropy alloys (HEAs) in their manufacture has attracted the most significant interest in creating tool materials with a self-healing effect today [68–70]. In particular, HEAs based on Fe-Co-Cr-Ni-Al and Co-Cr-Fe-Ni-Ti-Al are already effectively used as binders (mass fraction up to 20%) in the production of TiCN-TiB2 composite cermets by vacuum hot pressing. After receiving the samples, the binder structure based on the HEA is a solid solution, which tightly binds TiCN and TiB2. The use of the HEA makes it possible to obtain a fundamentally new tool material with a unique set of physical and mechanical properties compared to using a traditional binder (Ni-Co) in the manufacture of cermets. There is research on the creation of tungsten-containing hard alloys of the WC group using the Al-Co-Cr-Cu-Fe-Ni binder, which makes it possible to operate the material at higher thermal loads compared to samples in which Co is used as a binder [71]. The use of an HEA makes it possible to obtain a new class of tool material for a broader range of technological applications than traditional materials.

In addition, development is currently underway to use Fe-Co-Cr-Ni-Mo-based HEAs to create diamond and ceramic composites through spark plasma sintering and other advanced technologies [72–75]. The use of an HEA as a tool matrix in which diamond particles are embedded allows, by adjusting the technological conditions of sintering, various microstructures at the HEA/diamond interface and sufficient interfacial bond strength to be obtained. Researchers have discovered an interstitial strengthening effect, which has a very beneficial effect on the mechanical properties of the new tool material.
